An optical waveguide guides electromagnetic waves in the optical spectrum. Optical waveguides may be used as the transmission medium in optical communication systems. A waveguide can also be used as an optical amplifier, e.g. an Erbium-doped fiber amplifier. A planar waveguide (PWG) is a particular type of waveguide that guides an optical wave in only one transverse axis. A PWG has a planar, 3-layer sandwich geometry consisting of a higher refractive index middle (core) layer that is surrounded on both sides by lower refractive index cladding layers. A PWG typically has a high aspect ratio (e.g., 100:1 or more), i.e. thin in one transverse axis and wide in the other, and also possessing large flat surface areas that facilitate mounting and heat removal in certain configurations, e.g. PWG amplifiers. Light may be confined in the middle layer by total internal reflection since its refractive index is higher than the surrounding cladding layers. Guided modes of a PWG are excited by injecting light into one end of the core layer.
PWGs are often used in lasers, such as laser diodes. They are also used in many optical components, such as Mach-Zehnder interferometers and wavelength division multiplexers. The cavities of laser diodes are frequently constructed as rectangular optical waveguides.
A PWG amplifier is an optical amplifier that uses a waveguide to confine the optical signal, such as a laser beam, in a one-dimensional propagating mode thereby maintaining a high intensity in a long amplification path. Amplification is typically obtained by stimulated emission of photons from dopant ions in a doped core of the PWG. Typically, the core has a constant doping level. A pump laser excites ions into a higher energy level from where they can transition via stimulated emission of a photon at the signal wavelength back to a lower energy level. The excited ions can also decay spontaneously (spontaneous emission) or even through non-radiative processes involving interactions with phonons within the medium. These last two types of decay mechanisms compete with stimulated emission reducing the efficiency of light amplification. A major barrier to a high power laser gain medium (amplifier) is the maximum temperature along the device, which could lead to destruction or malfunctioning of the device. The temperature profile is at its peak where the pump light is input into the PWG. A high temperature gradient in a transverse direction also leads to wavefront distortion within the PWG. Power scaling is ultimately limited by thermal effects that are proportional to the peak heat load per unit length.
The dopant concentration has a direct effect on the performance of the PWG amplifier. Relatively high doping concentrations allow the desired signal amplification to occur using a PWG that is short in the direction of propagation, however this leads to relatively higher heating of the PWG which can cause it to degrade or fracture under thermal stress. High doping concentrations also produce more gain in the lateral direction giving rise to amplified spontaneous emission and parasitic lasing which quench the population inversion in the laser amplifier, thereby reducing the desirable signal amplification. Relatively low doping concentrations require longer PWGs to provide the desired signal amplification and these are more difficult to manufacture and handle.
A wavefront is the locus of points having the same phase, i.e., a line or curve in two dimensions, or a surface for a wave propagating in three dimensions. Wavefront distortion in a high power PWG amplifier results from thermal gradients in the unguided transverse axis of the PWG. Minimizing these transverse gradients helps enable high beam quality operation at high output power. Conventional high power PWG lasers use symmetric structures with cooling methods applied to both sides of the PWG.
Planar waveguides have historically been fabricated with uniform core thickness and uniform cladding thickness. This leads to higher heating near the end(s) where pump power is coupled into an end-pumped PWG. Since the power scaling is fundamentally limited by thermal effects, devices with non-uniform heating cannot achieve the power scaling potential of devices with more uniform heating. Prior attempts to improve thermal uniformity have utilized doping gradients or segmented doping in the gain medium to improve thermal uniformity but methods of fabricating laser gain media with doping gradients are low in maturity and complex to implement.